Major developments in magnetic resonance imaging (MRI) technology have been driven by the ever increasing demand for higher static magnetic field (B0) strengths. This increase, however, has posed many technical challenges, most notably the exacerbated inhomogeneity in both the main magnetic field (B0) and the radiofrequency (RF) magnetic field (B1). See, e.g., Blamire A M. The technology of MRI—the next 10 years? Brit J Radiol 2008; 81: 601-617; and Bernstein M A, Huston J, Ward H A. Imaging artifacts at 3.0 T. J Magn Reson Imaging 2006; 24:735-746.
A homogeneous B1 field can be required to ensure a uniform excitation across the sample. Recent advances in parallel excitation (also known as parallel transmit) technology have provided an effective means to address this issue by using a process termed “RF” or “B1” shimming, in which the amplitude, phase, timing, and frequency of the RF current in each coil element are independently adjusted. See, e.g., Vaughan T, DelaBarre L, Snyder C, Tian J F, Akgun C, Shrivastava D, et al. 9.4 T human MRI: preliminary results. Magn Reson Med 2006; 56:1274-1282; and Setsompop K, Wald L L, Alagappan V, Gagoski B, Hebrank F, Fontius U, Schmitt F, Adalsteinsson E. Parallel RF transmission with eight channels at 3 Tesla. Magn Reson Med 2006; 56:1163-1171. See also, U.S. Pat. Nos. 7,598,739 and 7,800,368, the contents of which are hereby incorporated by reference as if recited in full herein.
A homogeneous B0 field is required to ensure a correct spatial representation of the imaged object. Homogenization of the magnetic field distribution (i.e., B0 shimming) is often a difficult task when strong localized B0 inhomogeneities are present. See, e.g., Koch K M, Rothman D L, de Graaf R A. Optimization of static magnetic field homogeneity in the human and animal brain in vivo. Prog Nucl Magn Reson Spectrosc 2009; 54:69-96.
Passive shimming, which relies on the optimal arrangement of magnetized materials, is limited by the often tedious work required and the lack of flexibility in subject-specific conditions. See, Wilson J L, Jenkinson M, Jezzard P. Optimization of static field homogeneity in human brain using diamagnetic passive shims. Magn Reson Med 2002; 48:906-914.
On the other hand, active shimming, which utilizes continuously adjustable electromagnets, is the most widely used shimming method and typically employs spherical harmonic (SH) coils, including the ability to provide dynamic shimming. See, Golay M J, Field homogenizing coils for nuclear spin resonance instrumentation. Rev Sci Instrum 1958; 29:313-315; and Romeo F, Hoult D I. Magnet field profiling: analysis and correcting coil design. Magn Reson Med 1984; 1:44-65. And de Graaf R A, Brown P B, McIntyre S, Rothman D L, Nixon T. Dynamic shim updating (DSU) for multislice signal acquisition. Magn Reson Med 2003; 49:409-416. The contents of these documents are hereby incorporated by reference as if recited in full herein.
In practice, however, SH shimming often cannot effectively correct for high-order localized field distortions because the required number of coils increases dramatically with the SH order. See, Golay M J, Field homogenizing coils for nuclear spin resonance instrumentation, Rev Sci Instrum 1958; 29:313-315. As such, it is typically limited to the second or third order.
Recently, Juchem et al. have proposed a multi-coil modeling and shimming method, in which a large number of small localized electrical coils are used to shape the B0 field by independently adjusting the direct current (DC) in each coil, thus achieving an improved performance relative to SH shimming. However, it requires a separate set of shim coils adjacent to the RF coil array, which takes a considerable space within the constricted space between the subject and the magnet bore. In addition, when the shim coil array is placed within the RF coil array, a large gap needs to be kept open in the middle of the shim coil array to allow RF penetration and reduce the electromagnetic interference between the RF and shim coil arrays (i.e., RF damping), which reduces the flexibility and performance of the shimming. See, e.g., Juchem C, Nixon T W, McIntyre S, Rothman D L, de Graaf R A. Magnetic field modeling with a set of individual localized coils. J Magn Reson 2010; 204:281-289; Juchem C, Brown P B, Nixon T W, McIntyre S, Rothman D L, de Graaf R A. Multi-coil shimming of the mouse brain. Magn Reson Med 2011; 66:893-900; and Juchem C, Brown P B, Nixon T W, McIntyre S, Boer V O, Rothman D L, de Graaf R A. Dynamic multi-coil shimming of the human brain at 7 T. J Magn Reson 2011; 212:280-288. The contents of these documents are hereby incorporated by reference as if recited in full herein.